1. Field of the Invention
This invention relates to electric current interruption devices and, more particularly, to high voltage current limiting fuses of the type known as "full range."
2. Description of the Related Art
A fuse is a protective device for electric circuits which has a fusible element that melts and opens to interrupt the circuit when subjected to excessive current. The melting occurs, in large part, due to i.sup.2 R heating of the fusible element. A time delay occurs before circuit opening because the current which will cause the fusible element to operate or open must flow through the fusible element long enough for it to absorb sufficient heat to melt open. Thus, an important measure of the performance of a fuse is its time-current characteristic which is typically represented by a time-current curve which plots the logarithm of the time (in seconds) required for the fuse to operate versus the logarithm of the current in amperes. Other performance considerations include the continuous current rating which is the highest current the fuse can indefinitely pass without blowing, and the interrupting current rating which is the largest current that the fuse is capable of interrupting. For a given application, the value of the interrupting current rating should equal or exceed the largest current that the supply circuit is capable of providing.
A full range fuse is one which is capable of interrupting all values of current from its interrupting current rating down to the minimum continuous current that causes melting of the fusible element(s) with the fuse applied in a surrounding medium, in contact with the fuse, that is at the maximum temperature specified by the fuse manufacturer. One kind of such fuse employs two types of elements connected in a series electrical relationship. One element is primarily responsible for interrupting high fault currents, while the other element is primarily responsible for interrupting low fault currents, particularly very low currents. The ability of such a fuse to interrupt very low currents distinguishes "full range" fuses from other fuse types such as "back-up" and "general purpose" fuses.
U.S. Pat. No. 4,689,596 to Huber discloses a typical fuse employing the two element approach. The fuse includes a dielectric spider supporting a main or high fault current element and a secondary low fault current interruption fuse assembly connected in series therewith. The high fault current element is spirally wrapped around the dielectric spider and is electrically connected to one terminal cap. The low-fault current interruption fuse assembly is also spirally wrapped around the spider and is connected at one end to the high fault current element and at the other end to the other terminal cap. Such a fuse is referred to as the "dual element" type although the physical arrangement may involve more than two separate sections. For example, the high fault current element may be in two sections, with the low fault current assembly located between the two sections of the high fault current element. Such an arrangement is disclosed in U.S. Pat. No. 4,626,817 to Cameron.
A disadvantage of such fuses is their susceptibility to damage by a current surge having a magnitude in excess of the minimum current at which the high fault current element is designed to melt, yet of such short duration that the high fault current element only partially melts. Such damaging surges might be caused by transformer inrush currents, lightning surges, and the like. Should partial melting occur, the high fault current element may melt at a later time when the fuse is carrying a current less than the magnitude of current the high fault current element is designed to interrupt. Also, the fuse might be apt to fail to interrupt which, in turn, can result in the fuse catching fire and/or releasing ionized gas and flames with the potential for electrical flashover to adjacent energized parts. Thus, there is a need for a full range current limiting fuse with increased predictability of performance when subjected to electrical surges.